1. Field of the Invention
The present invention relates to a display apparatus and a method of driving a display panel, and more specifically, to a technique for correcting gradation data to adjust gradation of data displayed on a display panel.
2. Description of the Related Art
In a liquid crystal display, gamma correction is generally carried out, in which the correspondence between gradation data externally supplied and a driving signal for driving a display apparatus is corrected in accordance with the voltage-transmittance characteristic (V-T characteristic) of a liquid crystal panel. The V-T characteristic of the liquid crystal panel is nonlinear. Accordingly, in order to display an original image with a proper color tone, a nonlinear drive voltage for the gradation data needs to be generated through gamma correction. Further, to improve the color tone of the display image, the gamma correction may be carried out by using different gamma values for R (red), G (green), and B (blue), respectively. Since the voltage-transmittance characteristic of the liquid crystal panel differs among R (red), G (green), and B (blue), it is desirable that the gamma correction is carried out by using gamma values for the respective colors in order to improve the color tone of the display image.
In one of methods of achieving the gamma correction of the liquid crystal panel, data processing is carried out on gradation data. In this gamma correction, the data processing is carried out on input gradation data DIN in accordance with the following equation (1) and output gradation data DOUT is generated:DOUT=DOUTMAX(DIN/DINMAX)γ  (1)where DINMAX is the maximum value of the input gradation data and DOUTMAX is the maximum value of the output gradation data. The drive voltage signal for driving a signal line is generated in accordance with the generated output gradation data DOUT.
What is concerned with the gamma correction through the data processing is that the data processing includes repetitive multiplication such as power multiplication, as could be understood from the equation (1). Since a circuit becomes complicated to exactly perform power multiplication, a problem is caused when such a circuit is mounted on a liquid crystal driver. A CPU (Central Processing Unit) has excellent arithmetic capability, and the power multiplication can be exactly carried out through a combination of logarithmic calculation, multiplication, and exponential calculation by the CPU. For example, Japanese Laid Open Patent Application (JP-P2001-103504A) discloses gamma correction which is achieved through combination of the logarithmic calculation, multiplication, and exponential calculation. However, it is not preferable from the viewpoint of hardware reduction to mount the circuit for exact gamma correction on the liquid crystal driver.
In a simple method of accomplishing the gamma correction, a look-up table (LUT) is used in which the correspondence between input gradation data and output gradation data is described or defined in accordance with the equation (1). Thus, the gamma correction can be achieved without directly calculating the power multiplication. In Japanese Laid Open Patent Applications (JP-P2001-238227A and JP-A-Heisei 7-056545), the technique in which LUTs are provided for R (red), G (green), and B (blue), respectively, so that the gamma correction can be carried out for every gamma value for every color.
When the LUT is used for the gamma correction, increase in the size of LUT (or the number of LUTs) is required to perform the gamma correction to different gamma values. For example, if the gamma correction is carried out for each of R, G, and B, and for 256 kinds of gamma values by using an LUT in which the input gradation data is 6-bit data and the output gradation data is 8-bit data, the LUT of 393216 (=64×8×3×256) bits is required. This makes it difficult to incorporate the gamma correction circuit in the liquid crystal driver.
Japanese Laid Open Patent Application (JP-A-Heisei 9-288468) discloses a technique for carrying out the gamma correction to a plurality of gamma values while keeping the size of LUT small. In this conventional example, a rewritable LUT is provided in the liquid crystal display apparatus. Data to be held in the LUT is calculated by a CPU based on calculation data stored in an EEPROM and then transferred from the CPU to the LUT. Japanese Laid Open Patent Application (JP-P2004-212598A) also discloses a similar technique. In this conventional example, LUT data is generated by a brightness distribution determining circuit and the LUT data is transferred to the LUT.
Japanese Laid open Patent Application (JP-P2000-184236A) discloses a technique in which increase in the circuit size is suppressed by directly using the LUT not for the generation of output gradation data (the correspondence between the input gradation data and post-correction gradation data is described in the LUT) but for calculation of a parameter for broken line approximation of the gamma characteristic. In this conventional example, when a gamma value γ1 (a gamma value for a cathode-ray tube) for gamma correction carried out upon generation of input video data is given externally, a liquid crystal display apparatus generates broken line information for achieving the gamma correction on this input video data based on another gamma value γ2 (a gamma value for a liquid crystal display apparatus) by way of the broken line approximation. When the input video data is given, this liquid crystal display apparatus calculates the post-correction gradation data through the broken line approximation defined based on the broken line information.
One of demands on the liquid crystal display apparatus is instantly switching a gamma curve, that is, instantly switching a gamma value of gamma correction. For mobile terminals such as notebook type PCs, PDAs (Personal Digital Assistants), and cellular phones, due to their various possible usage environments, there is a demand to change the visibility of the liquid crystal panel in accordance with the environment. For example, in a liquid crystal display that uses a semi-transmissive LCD, an image is displayed mainly in a reflection mode when the intensity of external light is strong and mainly in a transparent mode when the intensity of external light is weak. Between the reflection mode and the transparent mode, the gamma value of the liquid crystal panel is different. Thus, the liquid crystal panel is viewed very differently depending on the intensity of external light. Therefore, the capability of instantly switching a gamma value allows a great improvement in the viewability of the liquid crystal display.
Another of demands is accurately performing the gamma correction with the simplest circuit. The equation (1) is based on the physical and physiological structure of human eyes. Therefore, a large difference of a value obtained from the exact equation (1) from the post-correction gradation data brings about an unnatural feeling of an image in the human vision. Therefore, ideally, it is desirable that the post-correction gradation data is coincident with the value obtained from an exact equation. However, the use of a complicated circuit for accurate gamma correction disadvantageously results in an increase in the cost of the liquid crystal driver. Therefore, accurate gamma correction by a simple circuit is one of major demands on the liquid crystal driver.
However, the conventional techniques fail to simultaneously satisfy these demands. For example, in the techniques described in Japanese Laid Open Patent Applications (JP-A-Heisei 9-288468 and JP-P2004-21259A), it is necessary to rewrite data to be stored in the LUT to the LUT for switching the gamma values of gamma correction. However, the data in the LUT has a considerable size. This means that it is difficult to instantly switch the gamma value of gamma correction.
On the other hand, as described in Japanese Laid Open Patent Application (JP-P2000-184286A), the method using broken line approximation suffers from difficulty in achieving accurate gamma correction.